1. Field of the Invention
The present invention relates to a three-position microswitch.
2. Description of the Related Art
The function of switching from an on state (ON) to an off state (OFF) may be performed by an electronic microcomponent such as a diode or a transistor. A major disadvantage of such microcomponents is that they exhibit on-state insertion losses and off-state leakage. To overcome this disadvantage, mechanical dual-position microswitches that limit insertion losses in closed position, i.e., in the on state, and exhibit a good isolation in open position, i.e., in the off state, may be used.
A conventional dual-position microswitch is shown in FIGS. 1A and 1B, where FIG. 1A shows a top view of the microswitch and FIG. 1B shows a cross-section view of the microswitch of FIG. 1A along line 1B—1B.
Microswitch 10 is formed on a substrate 12, for example, silicon, covered with an oxide layer 14, for example, silicon oxide. Oxide layer 14 comprises a parallelepiped-shaped cavity 16. The depth of cavity 16 is smaller than the thickness of oxide layer 14. A silicon nitride strip 18 extends over oxide layer 14 and spans cavity 16. The portion of silicon nitride strip 18 above cavity 16 forms a silicon nitride beam 20. In the absence of an external force, beam 20 has a convex shape so that it is at its farthest from the bottom of cavity 16 in its median portion.
Two conductive tracks 22, 24 extend on the bottom of cavity 16, substantially in prolongation of each other. The ends of conductive tracks 22, 24 are placed opposite to each other below beam 20. Two metal portions 26, 28 cover beam 20 close to its ends. Each metal portion 26, 28 forms with the portion of the underlying beam a structure that behaves as a bimetal. Two metal electrodes 30, 32 are arranged on the bottom of cavity 16 on either side of conductive tracks 22, 24 substantially below beam 20.
As shown in FIG. 1B, beam 20 comprises a contact pad 34 located on the surface of beam 20 opposite to the bottom of cavity 16. Two heating elements 36, 38 are comprised in beam 20 substantially opposite to metal portions 26, 28. Two complementary metal electrodes 40 and 42 are also comprised in beam 20 substantially opposite to electrodes 30, 32.
As shown in FIG. 1B, beam 20 takes at equilibrium a convex shape so that contact pad 34 is remote from conductive tracks 22, 24. Microswitch 10 then is in the off or open state.
To turn on microswitch 10, a current is run through heating elements 36, 38. The heat released by Joule effect causes a deformation of beam 20 that tends, in its central portion, to come closer to the bottom of cavity 16. The deformation is due to the expansion difference between metal portions 26, 28 and the areas of beam 20 around heating elements 36, 38, with metal portions 26, 28 expanding more. The expansion difference is sufficient to obtain the buckling of the central portion of beam 20.
FIG. 1C shows the microswitch after complete deformation of beam 20. Contact pad 34 is then in contact with both conductive tracks 22, 24. An electric connection between the two conductive tracks 22, 24 is thus obtained.
The supply of heating elements 36, 38 is then cut off. To maintain microswitch 10 on, a potential difference is applied between electrodes 30, 32 and complementary electrodes 40, 42. The resulting electrostatic forces tend to bring electrodes 30, 32 closer to complementary electrodes 40, 42, and to maintain pad 34 in contact with conductive tracks 22, 24.
In many applications, a microswitch with one off state and at least two on states is desired to be formed. For example, a microswitch with one off state and two on states having one input, a first output and a second output, is desired to be formed. Such a microswitch may have an off state corresponding to no connection between the input and the outputs, a first on state corresponding to the connection of the input to the first output and a second on state corresponding to the connection of the input to the second output.
To obtain a microswitch with one off position and two on positions that limits insertion losses and exhibits a good isolation, two dual-position microswitches of the type shown in FIGS. 1A to 1C may be combined. However, the obtained three-position microswitch takes up a significant space, generally at least the space taken up by the dual-position microswitch. Further, for same manufacturing technologies, the probability of obtaining such a three-position microswitch which is defective is greater than the probability of obtaining a dual-position microswitch which is defective. Further, it is necessary to double the electric control, especially the supply circuits of heating elements 36, 38.